add support for Tcl environment modules

First result from the 4th EasyBuild hackathon, required for lots of sites (e.g. JSC, DEISA systems, etc.)

Unit tests pass with this, both for C and Tcl environment modules:

```bash
$ export EASYBUILD_MODULES_TOOL=EnvironmentModulesTcl

$ eb PANDAseq-2.5-goolf-1.4.10.eb -ld | grep modulecmd.tcl
== 2013-10-22 17:04:42,929 main.EnvironmentModulesTcl DEBUG Running 'modulecmd.tcl python avail PANDAseq/2.5-goolf-1.4.10 -t' from /Users/kehoste/TMP/tcl...
== 2013-10-22 17:04:42,930 main.EnvironmentModulesTcl DEBUG Running module cmd 'modulecmd.tcl python avail PANDAseq/2.5-goolf-1.4.10 -t'

$ python -m test.framework.suite

....................................................................................................
----------------------------------------------------------------------
Ran 100 tests in 497.025s

OK

```